1. Field of the Invention
The present invention relates to distribution cylinders in printing machines, and more specifically to a distribution cylinder which incorporates a mechanism which enables the cylinder to move axially, backwards and forwards, at the same time as it rotates.
2. Description of the Related Art
Distribution cylinders are used in printing machines to smooth-out the ink layer on one or more printing contra-rotating rollers. By enabling the distribution cylinder to be driven by the contra-rotating printing roller or rollers at the same time as the cylinder is moved reciprocatingly in the direction of its long axis, the ink layer which ultimately meets the printing platen is smoothed-out or equalized. Poor equalization of the ink layer will result in print defects, such as striped print.
This axial, reciprocating movement of the distribution cylinder should be a uniform, sinusoidal movement whose frequency is coupled to the printing speed. This frequency depends on many machine factors, but normally often lies in the range of 0.5-2 Hz at normal printing speeds. The distribution cylinder should not vibrate at right angles to the cylinder surface, since such vibrations are liable to result in undesirable patterns in the ink layer to be equalized.
This axial movement of the cylinder is typically achieved with constructions that include levers, reduction gears and camming curves, all of which are fitted externally on the machine frame, within protective panels. It is also normal for each inking device to include up to four distribution cylinders. It will be understood that many mechanical mechanisms of the aforesaid kind are normally required to generate reciprocatory movement of all of such cylinders.
The incorporation of a mechanism within a distribution cylinder in order to achieve this axial movement is an old concept which has been applied in sheet-offset-printing machines. The distribution cylinders in these machines do not rotate as fast as the cylinders in modern web-offset-printing machines. Consequently, it is not necessary for the reduction of the ratio between cylinder speed and the frequency of the axial movement of the cylinder to be as great. A typical reduction in these known constructions is 9:1.
A known construction of this kind cannot be transferred to web-offset-printing machines, since the rollers of such machines rotate at high speeds and the low reduction ratio would then result in an axial movement frequency of such high magnitude as to risk the occurrence of harmful vibrations.
It is true that a satisfactory reduction ratio, which preferably lies in the range 30:1-40:1, could be achieved by incorporating multi-step gearboxes. Such a construction will afford a number of advantages, such as high efficiency, long useful life, long periods between servicing, low price, ease of exchange of the whole of the mechanism or parts thereof without disturbing the distribution cylinder, generally speaking independent of cylinder length.
A construction of this kind, however, also has serious drawbacks, such as:
a) Strict balancing requirements with regard to the complete distribution cylinder. In the case of cylinder diameters of about 75 mm, the normal maximum imbalance demand is about 6 gcm (gram-centimeter). PA0 b) Low gear mechanism efficiency. Heat emission along the cylinder, which influences the viscosity of the ink and therewith the quality of the resultant print. PA0 c) The load-carrying parts of the gear mechanism will have a short useful life, due to the large number of moveable parts and the play that occurs in time, coupled with relatively frequent services. PA0 d) The construction is also expensive, due to the large number of moveable parts. PA0 1) The reduction is too low. This means that axial movement of the cylinder will take place with an impermissibly high frequency, far above the desired frequency which, as before mentioned, lies in the range of 0.5-2 Hz. In order to obtain sufficiently high reduction, another gear solution is required, for instance the solution described in my U.S. Pat. No. 5,030,184. PA0 2) The x-y-linkage transfer mechanism is intended to transmit the rotary motion of the eccentrically journalled gearwheel to the rotational axle of the cam-curve unit. The mass of the x-y-linkage mechanism creates an imbalance, resulting in vibrations and frictional heat.
The German published specification No. 2 045 717 describes a distribution cylinder mechanism which comprises a single-step reduction gear and a cam-curve unit. The reduction gear is comprised of an eccentrically journalled gearwheel which meshes with an internally toothed annulus connected to the rotary cylinder. With the gear reduction possibilities available at that time, it was possible to achieve a maximum reduction of about 9:1 in this single step. The externally-toothed wheel journalled on the stationary eccentric transmits a slightly increased speed to the cam-curve unit, through the medium of an x-y-link mechanism.
The known distribution cylinder mechanism has two basic features which render it unsuitable for use in rapid, web-offset-printing machines, namely:
It is possible, of course, to substitute an x-y-linkage system with a universal drive shaft, diaphragm couplings or arcuate toothed couplings. This would make the construction more complicated, however, and therewith expensive. In addition, a construction of this kind would include many components which may become loose because of excessive play and therewith give rise to imbalance and vibrations.